Oil’s Rough Ride Returns for Investors

Saudi Clout Is No Guarantee Against Volatility

Oil has plunged into rough seas after an unusual period of calm. In that environment, investors should feel safer on a supertanker like Exxon Mobil than a skiff.

Recent sharp swings in oil prices could also work, eventually, to the advantage of a country seen by many as playing a central role in this: Saudi Arabia. Brent has dropped 22% in three months. The OPEC kingpin isn’t fond of getting fewer dollars for its biggest export. But instability is the enemy of planning. So while oil’s sharp moves could hamper the U.S....